[54.20] The UV Spectrum of the Ultra-compact X-ray Binary 4U 1626-67

We have obtained Hubble Space Telescope/STIS
low-resolution ultraviolet spectra of the X-ray pulsar 4U
1626-67 (=KZ TrA). This system is unusual even among X-ray
pulsars due to its ultra-short binary period (P=41.4 min)
and remarkably low mass-function (<1.3\times10-6Msun).
The 32ks far-UV spectrum reveals numerous broad emission and
prominent narrower absorption lines. Most of the absorption
lines are consistent in strength with a purely interstellar
origin, although CI and CIV appear to need additional
absorbing gas local to the system. Unusual abundance ratios
are also indicated by the emission lines; the usual
prominent lines of NV and HeII are absent, whilst both OIV
and OV are relatively strong and we see the unusual
~1660ÅOIII] multiplet. Our ultraviolet spectra
therefore provide independent support for the recent
suggestion that the mass donor is the chemically
fractionated core of either a C-O-Ne or O-Ne-Mg white dwarf;
as proposed to explain the results of Chandra
high-resolution X-ray spectroscopy. Furthermore, our
ultraviolet line profiles are in overall agreement with the
Doppler pairs found in the X-ray spectra, being in all cases
broad and/or flat-topped, or even double-peaked. Both the
X-ray and far-UV lines are plausibly formed in (or in an
corona just above) a Keplerian accretion disc; hence the
combination of ultraviolet and X-ray spectral data may
provide a rich data set for follow-on detailed models of the
disk dynamics and ionization structure in this highly
unusual low-mass X-ray pulsar system.

Support for this work was provided by NASA through STScI
grant GO-06624.01 and LTSA grant NAG5-7932.